Cooper Pairs in Insulators

Cooper pairs are the extraordinary link-up of like-charged electrons through the subtle flexings of a crystal. They act as the backbone of the superconducting phenomenon, but have also now been observed in a material that is not only non-superconducting but actually an insulator. An experiment at Brown University measures electrical resistance in a Swiss-cheese-like plank of bismuth atoms made by spritzing a cloud of atoms onto a substrate with 27-nm-wide holes spaced 100 nm apart. Bismuth films made this way are superconducting if the sample is many atom-layers thick but is insulating if the film is only a few atoms thick, owing to subtle effects which arise from the restrictive geometry.

The superconducting and insulating states are easily distinguished; as the temperature is lowered below the transition temperature (2 K) the resistance goes to zero for bismuth-as-superconductor, whereas for the insulating bismuth the resistance becomes extremely high. Cooper pairs are certainly present in the superconducting sample; they team up to form a non-resistive supercurrent. But how do the researchers know that pairs are present in the insulator too? Because of an additional test. By seeing what happens to resistance as an external magnetic field is increased.

The resistance should vary periodically, with a period proportional to the charge of the electrical objects in question. From the periodicity, proportional in this case to two times the charge of the electron, the Brown physicists could deduce that they were seeing doubly-charged objects moving through the sample. In other words, Cooper pairs are present in the insulator. This is true only at the lowest temperatures. One of the researchers, James Valles (james_valles_jr@brown.edu), says that there have been previous hints of Cooper pairs in some films related to superconductors, but that in those cases the evidence for pairs in the insulating state was ambiguous, and not as direct as the observation recorded in the Brown lab. He asserts that the realization of a boson insulator (in which the charge carriers are electron pairs) will help to further explore the odd kinship between insulators and superconductors. (Stewart et al., Science 23 November 2007)